2000 Honda Civic multiple/random misfires

I’ve got a 2000 Honda Civic EX I acquired recently that was parked for a couple years and it’s throwing codes for misfires on all cylinders and a multiple random misfire code. P0300, P0301, P0302, P0303, P0304. It is most prevalent at idle, but with any sort of load applied (a/c, lights, p/w etc.) the misfire is much less noticeable audibly. At 1/2 to WOT or essentially when it is in open loop operation it runs smooth. It is not a dead miss.


When I got the car it did not run, there was poor spark. I have replaced the coil with a new NGK coil, NGK plugs, NGK wires, Dist cap, Rotor. I have cleaned the IAC valve, replaced the PCV Valve, as it was stuck/clogged, checked for vacuum leaks(there are none), cleaned engine/battery ground(s), performed a valve adjustment, replaced the air filter and fuel filter, ran injector cleaner in gas tank and a seafoam top engine cleaner. The car also has a brand new battery and alternator. I also have replaced the catalytic converter, as well as installed both new Upstream and downstream Oxygen sensors when I replaced the Catalytic converter (OEM cat was completely clogged from prior neglect, was ran with major exhaust leak at header flange) The O2 sensor graph is a nice wave from .1 - .9 V and the STFT is usually between +/- 5% at idle, nothing excessive, even though I can hear, see and smell it running rich. when I get it to 4K RPMs or higher at low throttle, it gets very rich. I can see black smoke from exhaust as well as smell the fuel. STFT will go to -30% and higher under the right condition. The MAP and TPS both read good values when compared to my other good running civic of identical year/trim. ECT and IAT also read correct values. If the car is warmed up and misfiring and I shut it off and restart it, it will not misfire until the ECU switches into Closed loop operation after about 10 seconds. This makes me think it was an oxygen sensor but those have been replaced, and even so it doesn’t appear to be making the proper corrections, it is pulling fuel but not enough to smooth it out. Compression numbers were good, all between 175-195. And I have no coolant or oil mixing in either my radiator or crankcase and no white or blue smoke visible that would indicate a bad head gasket. it’s not using any fluids other then a small oil leak from the oil pan and a front main seal. Obviously I am missing something but this is about as far as I can go without some higher level knowledge on where to look.

I follow that the compression numbers are good but with older Honda and the symptoms you describe valve clearances are always suspect.

I was suspect of valve clearances at one point during the diagnosis and did perform a valve adjustment with no change in operation observed. I know I had a long post and that probably got lost in it. This is sucking down fuel like a diesel truck. I’m blowing clouds of black smoke and during the 1 tank of fuel I have ran though it, I averaged 16mpg…. This car should be in the mid 30’s. There is something causing it to way over fuel.

Anyone have any other ideas ? It would be greatly appreciated

Does this engine have an EGR valve?
Typically, unwanted EGR would skew the MAP, which you say is reporting the same as a known good car you have. So, that may not be your problem, but I think considering the work you have into it that it would be worth checking. I have had success simply unbolting the EGR valve and blocking the passage with a coin, then bolting the EGR valve back on.

I wish this car had an EGR just so I could fix it and be done, but sadly this trim isn’t equipped with EGR. There are many similar era civics with EGR that cause similar issues, but the 96-00 EX trim does not have an EGR system.
I am going to take off and clean the Throttle body today, I noticed it’s a little dirty so I’m just going to check it off the list, I don’t have my hopes too high though.

Well, I just got out of the garage for the evening and was able to get at this a bit. I first completely removed the throttle body and gave it a thorough cleaning. It had about .5-1.0mm of buildup on the walls around the plate. I was able to get the main inlet and all passages perfectly clean. I also removed the IACV again and re-cleaned it.

I got everything bolted back up with new gaskets and fired it up. It was getting more air naturally so it was idling higher then before. I let it fully warm up and then performed the idle relearn procedure. Unplug the IACV, set idle screw to approx 700rpm. Reset ECU, etc. it’s idling at the correct RPM, but it’s still misfiring and running rich. It may be pulling slightly less on the STFT’s, maybe -27% vs -31% before, but really, not much change.

I also went over all vacuum hoses anywhere on the engine and pulled them. Any vacuum lines are pulling vacuum as required and not leaking or drawing in any in-wanted or in-metered air.

When the engine is cool and I unplug the IACV the rpms will drop as one would expect. This tells me the valve is opening and closing appropriately. I have no hunting idle. Are there any more technical tests for this? The worst of my problem seems to occur at around 3-4K rpm at high vacuum levels, when there is very low throttle opening. Essentially the middle to upper range of nearly every shift change for normal, non-spirited driving..